Electric power systems need to provide electric power in a reliable fashion. Therefore such systems often comprise backup devices, for example in form of DC power sources. The backup devices may provide power for evening out fluctuations, shortages etc.
An example of such a device, usable as a backup, is a battery energy storage as illustrated in FIG. 3. The battery energy storage 10 comprises a number of series- and/or parallel-connected battery cells arranged in battery modules 20. The battery modules 20 in turn are series- and/or parallel-connected to form a battery unit 30 and several battery units 30 may be series-connected to form a battery string 40. In large battery energy storage systems, several such battery strings 40 are connected in parallel between negative and positive busbars. The battery strings 40 are connected to a load, for example a converter system 50, which in turn is connected to a power network transmission system. Circuit breakers 60 may further be arranged between each battery string and the respective DC busbars.
WO2008/002223 A1 describes a power compensator (see abstract) for an AC electric power transmission. The power compensator comprises a voltage source converter (4 in the figure) connected to a capacitor means (6) in parallel with a DC energy storage device (5). The energy storage device (5) comprises a high voltage battery means, including a battery string, having a short circuit failure mode, a first (40a) switch and a second switch (40b) for disconnecting the battery string, and a control unit (44) for operating these switches (40a, 40b).
A drawback with these short circuit protection switches is that they have to be dimensioned for high voltage, including large space requirements, and must be able to break very large short circuit currents.